For an image forming device such as a printer or a photocopier, one of the major problems to be achieved is reduction of entire power consumption by further decreasing power consumed at the stand-by state. Therefore, some image forming devices adopt a power supply device that stops power supply from the main power source circuit at the standby state in terms of reduction of entire power consumption.
However, in some cases, the device in the standby state needs to be back to the general operation in response to an externally supplied signal. For example, a photocopier with a facsimile function needs to be ready for receipt of externally supplied Fax data etc. that is transmitted via a telephone line. Further, for example, a printer needs to be back to the general operation immediately in response to detection of image data supplied from a personal computer etc., so as to carry out image forming according to the image data.
Under such circumstances, some prior art documents disclose a computer system and a control method for realizing low power consumption by driving the device under a low power mode while allowing the device to be back to the general mode any time necessary (e.g. Japanese Laid-Open Patent Application Tokukai 2000-32081 (published on Jan. 28, 2000)). The computer system and the control method for low power consumption disclosed in Document 1 are realized by software that detects data communication condition of the system using an OS or the like, so as to prevent the system from shifting to the low power mode during the data communication mode.
However, in conventional techniques such as Document 1, the shift to the low-power mode is mainly prevented by software such as an OS, and therefore, the system is vulnerable to malfunction of the program constituting the software, thus failing to thoroughly ensure secure operation.
Further, due to the time taken to cause the CPU to enforce the program constituting the software, the operation speed may decrease.